Software refreshed memory device and method

ABSTRACT

A software refreshed memory device comprises a plurality of memory cells that must be periodically refreshed to avoid losing data. Preferably, the memory cells can avoid losing data even though the time interval between successive memory refresh operations is relatively long, as compared to the time interval between successive memory refresh operations in a conventional volatile memory device, such as a DRAM. A processor can perform periodic memory refresh operations by executing a set of memory refresh instructions implemented in software, rather than in hardware. Accordingly, the memory device can advantageously be simplified, because the need for memory refresh circuitry and for a unique refresh control signal are advantageously eliminated. Moreover, the processor executing the memory refresh instructions can typically perform more sophisticated algorithms, as compared to memory refresh circuitry implemented in hardware, for determining when to perform a memory refresh operation. For example, the processor can determine whether each individual memory cell needs to be refreshed, thereby advantageously avoiding performing unnecessary refresh operations on memory cells that do not need to be refreshed.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.11/287,488, now U.S. Pat. No. 7,307,908, filed on Nov. 28, 2005, whichin turn is a continuation of U.S. application Ser. No. 10/234,001, nowU.S. Pat. No. 7,010,644, filed on Aug. 29, 2002, the disclosures ofwhich are herewith incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to computer memory devices and moreparticularly to memory devices that must be periodically refreshed toavoid losing data.

2. Description of the Related Art

Certain memory devices can maintain the information stored in the memoryindefinitely, even when the power to the memory device is turned off.These memory devices are known as non-volatile memory devices. Someexamples of non-volatile memory devices include magnetic random accessmemories (MRAMs), erasable programmable read only memories (EPROMs), andvariations thereof.

Other memory devices require power to maintain the information stored inthe memory. These memory devices, which are known as volatile memorydevices, must be periodically refreshed to avoid losing data. One commonexample of a volatile memory device is a dynamic random access memory(DRAM), wherein voltages stored in capacitors represent digital bits ofinformation. Because the voltage stored in a capacitor dissipates overtime, the capacitors of a DRAM must be periodically re-charged tomaintain the information stored in the DRAM.

Conventional volatile memory devices include a plurality of individualmemory cells configured in an array, which typically comprises aconfiguration of intersecting rows and columns. To maintain theinformation stored in a conventional volatile memory device, each memorycell in the array is typically refreshed at least several times persecond. For example, in some conventional DRAMs, each memory cell mustbe refreshed about once every 64 milliseconds to avoid losing theinformation stored in the DRAM. Accordingly, conventional volatilememory devices typically include refresh circuitry to ensure that everymemory cell is refreshed at least as often as necessary to avoid losingdata, which is commonly at least several times per second.

SUMMARY OF THE INVENTION

In one embodiment, a method of refreshing a cell in a memory devicecomprises determining whether said cell is in a written state or in anerased state, and, if said cell is in said written state, refreshingsaid cell by issuing a refresh write instruction to said cell.

In another embodiment, a method of preserving data stored in a volatilememory device having a plurality of cells comprises, for each of saidplurality of cells, determining whether said cell needs to be refreshed,and, if said cell needs to be refreshed, refreshing said cell.

In another embodiment, a method of preserving data stored in a volatilememory device having a plurality of cells comprises refreshing saidplurality of cells, and waiting for a predetermined period of timelasting for at least about one second, wherein none of said plurality ofcells is refreshed during said predetermined period of time.

In another embodiment, a method of preserving data stored in a volatilememory device having a plurality of memory cells comprises addressing afirst memory cell, waiting for a first period of time since said firstmemory cell was addressed, and determining whether said first memorycell needs to be refreshed. If said first memory cell needs to berefreshed, the method further comprises determining whether systemresources are available to refresh said first memory cell, and, if saidfirst memory cell needs to be refreshed and if said system resources arenot available, monitoring whether said system resources become availableto refresh said first memory cell within a second period of time sincesaid first memory cell was addressed. If said first memory cell needs tobe refreshed and if said system resources do not become available withinsaid second period of time, the method further comprises forcing saidresources to be relinquished, such that said resources become availableto refresh said first memory cell, and, if said first memory cell needsto be refreshed, refreshing said first memory cell using said availablesystem resources. The method further comprises addressing a secondmemory cell.

In another embodiment, a method of avoiding loss of data in a volatilememory device comprises establishing a deadline by which said volatilememory device must be refreshed, and monitoring whether resources areavailable to refresh said volatile memory device. If resources do notbecome available to refresh said volatile memory device within a firstpredetermined time period before said deadline, the method furthercomprises forcing said resources to be relinquished, such that saidresources become available to refresh said volatile memory device. Themethod further comprises using said available resources to refresh saidvolatile memory device before said deadline.

In another embodiment, a computer system comprises a processor, a memorydevice coupled to said processor, wherein said memory device comprises aplurality of cells that must be periodically refreshed, and a softwaremodule that, when executed by said processor, refreshes said pluralityof cells.

In another embodiment, a memory device comprises a plurality of cellsthat must be periodically refreshed, wherein said plurality of cells areconfigured to be refreshed in response to a write instruction or anerase instruction received from a processor, and wherein said memorydevice is not configured to generate or to receive a refresh controlsignal that differs from said write instruction or from said eraseinstruction.

In another embodiment, a memory device comprises a plurality of cellsthat must be periodically refreshed, wherein said plurality of cells areconfigured to be refreshed in response to a write instruction or anerase instruction received from a processor, and wherein said memorydevice is not configured to generate or to receive a refresh controlsignal that differs from said write instruction or from said eraseinstruction. Each of said plurality of cells comprises a programmablemetallization cell, which comprises a cell body having a top surface,wherein said cell body comprises a chalcogenide-metal ion glass and twoelectrodes disposed at said top surface, wherein said electrodes arespaced a distance apart from one another.

In another embodiment, a memory device comprises a plurality of cellsthat must be periodically refreshed, wherein said plurality of cells areconfigured to be refreshed in response to a write instruction or anerase instruction received from a processor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a computer system that includes a volatile memorydevice.

FIG. 2 illustrates a computer system having a software refreshed memorydevice in accordance with one embodiment of the present invention.

FIG. 3 illustrates a method of refreshing a memory cell in accordancewith one embodiment of the present invention.

FIG. 4 illustrates a method of refreshing a plurality of memory cells inaccordance with one embodiment of the present invention.

FIG. 5 illustrates a method of refreshing a plurality of memory cells inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a computer system 100 that includes a conventionalvolatile memory device 110. The computer system 100 also comprises aprocessor 120 coupled to the memory device 110 via a bus 130. The memorydevice 110 comprises an address/data/control module 140 and memoryrefresh circuitry 150, both of which are coupled to the processor 120via the bus 130. The memory device 110 also comprises a memory array 160coupled to the address/data/control module 140 via line 200 and to thememory refresh circuitry 150 via line 210. Those of ordinary skill inthe art will understand that lines 200 and 210 may be implemented in thememory device 110 as part of a single physical bus. The memory array 160comprises a plurality of memory cells 170, which are interconnected by aplurality of intersecting rows 180 and columns 190.

To perform operations in the memory device 110, the processor 120transmits certain signals to the memory device 110 via the bus 130. Forexample, to read data stored at a particular memory address, theprocessor 120 issues a read command, together with the memory address,to the bus 130. The address/data/control module 140 receives andprocesses the read command by accessing the memory array 160 via line200. Specifically, the address/data/control module 140 generates a readcontrol signal on line 200 and addresses the desired memory cell 170 byactivating the appropriate row 180 and column 190. Theaddress/data/control module 140 then receives the data stored at theaddressed memory cell 170 via line 200, and passes the data to theprocessor 120 via the bus 130.

In addition, to write data to a particular memory address, the processor120 issues a write command, together with the memory address and thedata to be stored, to the bus 130. The address/data/control module 140receives and processes the write command by generating a write controlsignal on line 200 and addressing the desired memory cell 170, asdescribed above. The address/data/control module 140 then passes thedata to be stored to the addressed memory cell 170 via line 200.

Because the data stored in the memory cells 170 dissipates over time,the memory cells 170 must be periodically refreshed to avoid losing thedata stored in the memory device 110. The primary function of the memoryrefresh circuitry 150 is to perform these periodic memory refreshoperations. When the memory refresh circuitry 150 determines that it isnecessary to perform a memory refresh operation, the memory device 110is made unavailable to the processor 120 to perform other operations,such as a read operation or a write operation.

To perform a memory refresh operation, the memory refresh circuitry 150typically sequentially addresses the rows 180 of the memory array 160.When a given row 180 of the array 160 is addressed, the memory refreshcircuitry generates a refresh control signal on line 210, which causesall of the memory cells 170 in the addressed row 180 to be refreshedsimultaneously. By refreshing a large number of memory cells 170simultaneously, the memory refresh circuitry 150 advantageously reducesthe amount of time required to perform a memory refresh operation,thereby reducing the amount of time that the memory device 110 isunavailable to the processor 120 to perform other operations.

Because the memory device 110 is unavailable to perform other operationsduring a memory refresh operation, it is desirable to time memoryrefresh operations such that they occur at times when they will notconflict with requests from the processor 120. Therefore, the memoryrefresh circuitry 150 is often configured to identify time periods whenthe processor 120 is not likely to issue requests to the memory device110, and to perform memory refresh operations during these identifiedtime periods. On the other hand, because power is required to perform amemory refresh operation, it is desirable to minimize the number ofmemory refresh operations performed. Thus, the memory refresh circuitry150 is also often configured to perform memory refresh operations asinfrequently as possible, while preserving the data stored in the memorydevice 110.

Nevertheless, to avoid losing data stored in conventional volatilememory devices 110, the memory refresh circuitry 150 must perform memoryrefresh operations frequently, often many times per second. For example,in some embodiments, each memory cell 170 must be refreshed at leastabout once every 64 milliseconds to avoid losing data. In theseembodiments, to avoid losing the information stored in the memory device110, the memory refresh circuitry 150 must ensure that every memory cell170 is refreshed at least more often than about 15 times per second.

FIG. 2 illustrates a computer system 250 having a software refreshedmemory device 260 in accordance with one embodiment of the presentinvention. The computer system 250 also comprises a processor 270coupled to the memory device 260 via a bus 280. The computer system 250further comprises a set of memory refresh instructions 290, which areimplemented in software that can be executed by the processor 270. Thememory device 260 comprises an address/data/control module 300 which iscoupled to the processor 270 via the bus 280. The memory device 260 alsocomprises a memory array 310 coupled to the address/data/control module300 via line 350. The memory array 310 comprises a plurality of memorycells 320, which are interconnected by a plurality of intersecting rows330 and columns 340.

The computer system 250 illustrated in FIG. 2 can perform memory readand write operations using the same methods described above inconnection with FIG. 1. In addition to these methods, however, thecomputer system 250 can perform memory read and write operations in avariety of other ways that are well-known to those of ordinary skill inthe art.

In some embodiments, the memory cells 320 of the software refreshedmemory device 260 comprise volatile memory cells, which are preferablymore stable than conventional DRAM memory cells. For example, in oneembodiment, the memory cells 320 can avoid losing data even though thetime interval between successive memory refresh operations is about 0.1seconds. In another embodiment, the time interval between successivememory refresh operations can be about one second. In yet anotherembodiment, the time interval between successive memory refreshoperations can be about one hour. In yet another embodiment, the timeinterval between successive memory refresh operations can be about oneday to one week.

In some embodiments, the memory cells 320 of the software refreshedmemory device 260 comprise programmable conductor random access memory(PCRAM) cells, which are described in U.S. Pat. Nos. 5,761,115,5,896,312, 5,914,893, 6,084,796 to Kozicki et al. (“the Kozickipatents”), in U.S. Pat. No. 6,348,365 to Moore et al. (“the Moorepatent”), and in the following co-pending U.S. patent applications: Ser.No. 10/121,792 entitled “Method of Manufacture of Programmable ConductorMemory” filed Apr. 10, 2002, U.S. Ser. No. 10/121,790 entitled“Programmable Conductor Memory Cell Structure and Method Therefor” filedApr. 10, 2002, and Ser. No. 10/121,794 entitled “Thin Film DiodeIntegrated with Chalcogenide Memory Cell” filed Apr. 10, 2002. TheKozicki patents, the Moore patent, and these co-pending patentapplications are hereby incorporated in their entireties by thisreference. As discussed in more detail in these references, a PCRAM cellcomprises a pair of electrodes and can exist in one of two possiblestates. In the first state, an electrical short exists between theelectrodes of the PCRAM cell. In the second state, an open circuitexists between the electrodes of the PCRAM cell.

Some embodiments of a PCRAM cell comprise a glass ion conductor, such asa chalcogenide-metal ion glass, and two electrodes disposed at thesurface of the glass ion conductor and spaced a distance apart from oneanother. In one embodiment, a PCRAM cell comprises germanium selenidewith a Group IV metal (e.g., silver) dissolved therein, such asAg/Ge₃Se₇. Preferably, one of the electrodes comprises a Group IV metal,and the glass element of the PCRAM cell contains the same metal.

In operation, when a voltage having a first polarity is applied acrossthe electrodes of a PCRAM cell, a conductive path is created between theelectrodes along the sidewalls of the via in which the glass element isformed. When a voltage having the opposite polarity is applied acrossthe electrodes, the metal ions re-dissolve into the cell body, therebycausing the conductive path to disappear. The presence or absence of aconductive path within a PCRAM cell can be detected by measuring theelectrical resistance between the electrodes. When a conductive path ispresent, an electrical short exists between the electrodes, and theresistance between the electrodes is low (e.g., on the order ofmilliohms). On the other hand, when no conductive path is present, anopen circuit exists between the electrodes, and the resistance betweenthe electrodes is high (e.g., on the order of megaoluns).

The features of FIG. 2 are referenced throughout the discussion below ofoperation processes.

Typically, the memory cells 320 of the memory device 260 are capable ofexisting in one of two states, i.e., a “written” state or an “erased”state. For example, if a memory cell 320 comprises a capacitor capableof holding a charge, the presence of a charge in the capacitor wouldcorrespond to the written state, and the absence of a charge in thecapacitor would correspond to the erased state. Similarly, if a PCRAMcell acts as a memory cell 320, the presence of a conductive pathbetween the electrodes would correspond to the written state, while theabsence of a conductive path between the electrodes would correspond tothe erased state. Those of ordinary skill in the art will understandthat, in general, the presence of an element of interest within a memorycell 320 will correspond to the written state, whereas the absence of anelement of interest will correspond to the erased state.

As discussed above in connection with FIG. 1, when a memory cell 170 ofa conventional volatile memory device 110 is placed in a particularstate, the memory cell 170 remains in the given state for a relativelyshort period of time, such as, for example, about 64 milliseconds.Because such conventional memory cells 170 retain their assigned statesfor such a short time period, each memory cell 170 must be refreshedoften, such as, for example, at least more often than about 15 times persecond.

By contrast, when a memory cell 320 of the software refreshed memorydevice 260 illustrated in FIG. 2 is placed in a particular state, thememory cell 320 advantageously remains in the given state for arelatively long period of time. For example, in some embodiments, eachmemory cell 320 can maintain a given state for a period of seconds,minutes, hours, days, weeks, or longer. Accordingly, the data stored inthe memory device 260 can be preserved while performing memory refreshoperations less frequently, such as, for example, about once every fewweeks, rather than several times per second.

Because memory refresh operations can occur less frequently, thecomputer system 250 can advantageously perform these memory refreshoperations by executing a set of memory refresh instructions 290implemented in software, rather than in hardware. For example, in someembodiments, the memory refresh instructions 290 constitute part of theoperating system of the computer system 250.

By implementing the memory refresh instructions 290 in software, ratherthan in hardware, the memory device 260 can advantageously besimplified. For example, the need for the memory refresh circuitry 150and for a unique refresh control signal on line 210, as illustrated inFIG. 1, are advantageously eliminated. Another advantage of implementingthe memory refresh instructions 290 in software, rather than inhardware, is that the processor 270 (FIG. 2) can perform moresophisticated algorithms, as compared to the memory refresh circuitry150 (FIG. 1), for determining when to perform a memory refreshoperation.

FIG. 3 illustrates a method of refreshing a memory cell 320 (FIG. 2) inaccordance with one embodiment of the present invention. In a first step400, the process begins. In a next step 402, the processor 270 reads thestate of the memory cell 320. The state of the memory cell 320 isassigned a logical value, which corresponds to a digital bit of data.For example, in some embodiments, the written state of a memory cell 320may correspond to a logical “1”, while the erased state of the memorycell 320 corresponds to a logical “0”. In some alternative embodiments,on the other hand, the written state of a memory cell 320 may correspondto a logical “0”, while the erased state of the memory cell 320corresponds to a logical “1”.

In a step 404, the processor 270 determines whether the data bit storedin the memory cell 320 is a “1” or a “0”. If the data bit is a “1”,then, in a step 406, the processor 270 writes a “1” to the memory cell320. For example, if a “1” corresponds to the written state, then,during step 406, the processor 270 issues a “write instruction” to thememory cell, i.e., a write command is issued, and the memory cell 320 isplaced in the written state. If, during step 404, the processor 270determines that the data bit stored in the memory cell 320 is a “0”,then, in a step 408, the processor 270 writes a “0” to the memory cell320. For example, if a “0” corresponds to the erased state, then, duringstep 408, the processor 270 issues an “erase instruction” to the memorycell, i.e., a write command is issued, and the memory cell 320 is placedin the erased state.

A write instruction issued during a memory refresh operation can be thesame as a write instruction issued during a standard write operation tothe memory device 260. Similarly, an erase instruction issued during amemory refresh operation can be the same as an erase instruction issuedduring a standard write operation to the memory device 260. Accordingly,as discussed above, the need for a unique refresh control signal, whichis utilized only during memory refresh operations, is advantageouslyeliminated. After the processor 270 completes step 406 or step 408,then, in a final step 410, the process ends.

In the method illustrated in FIG. 3, it is assumed that a memory cell320 (FIG. 2) will not remain in the written state or in the erased stateindefinitely unless it is periodically refreshed by issuing a writeinstruction or an erase instruction, whichever is appropriate, to thememory cell 320. In some embodiments, however, the memory cells 320 tendto revert to one particular state over time. For example, in someembodiments, the memory cells 320 tend to revert to the erased stateover time. In these embodiments, when a memory cell 320 is placed in thewritten state, the memory cell 320 will not remain in the written stateindefinitely unless it is periodically refreshed by performing a writeoperation on the memory cell 320. On the other hand, because the naturaltendency of the memory cells 320 is to revert to the erased state, amemory cell 320 placed in the erased state will remain in this stateindefinitely, without needing to be refreshed. In these embodiments, themethod illustrated in FIG. 3 can advantageously be simplified byeliminating either step 406 or step 408, whichever corresponds to theerased state.

Moreover, in the method illustrated in FIG. 3, it is assumed that eachmemory cell 320 (FIG. 2) is capable of existing in one of only twostates. Nevertheless, in light of the present disclosure, those ofordinary skill in the art will understand how the method illustrated inFIG. 3 can be modified if the memory cells 320 are capable of existingin more than two states.

If a memory refresh operation comprises the method illustrated in FIG.3, then each memory cell 320 (FIG. 2) is refreshed individually; ratherthan refreshing a large number of memory cells 320 simultaneously, asdiscussed above in connection with FIG. 1. By evaluating each memorycell 320 individually, the processor 270 can determine whether eachindividual memory cell 320 needs to be refreshed, thereby advantageouslyavoiding performing unnecessary refresh operations on memory cells 320that do not need to be refreshed.

FIG. 4 illustrates a method of refreshing a plurality of memory cells320 (FIG. 2) in accordance with one embodiment of the present invention.To perform this method, the computer system 250 maintains a counterhaving a value which corresponds to a particular address in the memoryarray 190. In a first step 450, the memory cell 320 at the addresscorresponding to the current value of the counter is refreshed, usingthe method illustrated in FIG. 3 or another suitable method. In a nextstep 452, the value of the counter is incremented, and in a next step454, a timer is reset and started.

In a step 456, the processor 270 determines whether the timer hasexceeded a predetermined minimum wait time. The appropriate value forthe minimum wait time can be determined by considering a number offactors, such as, for example, the maximum time that a memory cell 320can retain its assigned state, the time required to refresh a memorycell 320, the number of memory cells 320 to be refreshed, and the like.As discussed above, this predetermined minimum wait time canadvantageously be a relatively long period of time, such as, forexample, a period of seconds, minutes, hours, days, or longer. In oneembodiment, the predetermined minimum wait time is a period of about oneminute. In another embodiment, the minimum wait time is a period ofabout one hour. In yet another embodiment, the minimum wait time is aperiod of about one day to one week.

If the minimum wait time has not yet been reached, then, in a step 458,the processor 270 determines whether the memory cell 320 at the addresscorresponding to the current value of the counter needs to be refreshed.A number of different conditions may indicate that the memory cell 320at the current address does not need to be refreshed. For example, asdiscussed above, in some embodiments, when a memory cell 320 is in theerased state, the memory cell 320 does not need to be refreshed.Moreover, if the processor 270 performs a write operation to the memorycell 320 sometime after the timer is reset and started during step 454,then the memory cell 320 does not need to be refreshed until the nextmemory refresh cycle.

If, while waiting for the timer to reach the minimum wait time, theprocessor 270 determines that the memory cell 320 at the current addressdoes not need to be refreshed, then the process returns to step 452,where the value of the counter corresponding to the current memoryaddress is incremented, and the process continues, as described above.On the other hand, if, once the timer reaches the minimum wait time, thememory cell 320 at the current address still needs to be refreshed, thenthe process proceeds to a step 460, where the processor 270 determineswhether system resources are available to refresh the memory cell 320.In making this determination, the processor 270 may evaluate a widevariety of factors, such as, for example, the demands on the processor270 and on the memory device 260 or other devices in the computer system250, and the like.

If system resources are available, then the process returns to step 450,where the memory cell 320 at the current address is refreshed, and theprocess continues, as described above. On the other hand, if systemresources are not available to refresh the memory cell 320, then, in astep 462, the processor 270 determines whether the timer has exceeded apredetermined maximum wait time. As with the minimum wait time, theappropriate value for the maximum wait time can be determined byconsidering a number of factors, such as, for example, the maximum timethat a memory cell 320 can retain its assigned state, the time requiredto refresh a memory cell 320, the number of memory cells 320 to berefreshed, and the like. As discussed above, this predetermined maximumwait time can advantageously be a relatively long period of time, suchas, for example, a period of seconds, minutes, hours, days, weeks, orlonger. In one embodiment, the maximum wait time is a period of aboutone hour. In another embodiment, the maximum wait time is a period ofabout one week. In yet another embodiment, the maximum wait time is aperiod of about one month.

If the maximum wait time has not yet been reached, then, in a step 464,the processor 270 determines whether the memory cell 320 at the addresscorresponding to the current value of the counter needs to be refreshed.As discussed above in connection with step 458, a number of differentconditions may indicate that the memory cell 320 at the current addressdoes not need to be refreshed.

If the memory cell 320 at the current address does not need to berefreshed, then the process returns to step 452, where the value of thecounter corresponding to the current memory address is incremented, andthe process continues, as described above. On the other hand, if thememory cell 320 at the current address needs to be refreshed, then theprocessor 270 continues to monitor whether system resources have becomeavailable to refresh the memory cell 320.

Once the timer reaches the predetermined maximum wait time, if systemresources have not become available and the memory cell 320 at thecurrent address still needs to be refreshed, then the process proceedsto a step 466, where the processor 270 forces certain system resourcesto be relinquished by other processes, such that the necessary resourcesbecome available to refresh the memory cell 320. The process thenreturns to step 450, where the memory cell 320 at the current address isrefreshed, and the process continues, as described above.

In one embodiment, the process illustrated in FIG. 4 is repeatedlyperformed, without interruption, by the processor 270. In thisembodiment, the memory refresh operation is an ongoing process, which isconstantly occurring in the background of other processes being executedby the processor 270. The minimum wait time and the maximum wait timecan advantageously be selected and adjusted such that each memory cell320 is refreshed as infrequently as possible, while preserving theinformation stored in the memory device 310.

FIG. 5 illustrates another method of refreshing a plurality of memorycells 320 in accordance with one embodiment of the present invention. Ina first step 500, the process begins, and in a next step 502, a timer isreset and started. As with the method illustrated in FIG. 4, thecomputer system 250 of FIG. 2 maintains a counter having a value whichcorresponds to a particular address in the memory array 310 to performthe method illustrated in FIG. 5. In a step 504, the memory cell 320 atthe address corresponding to the current value of the counter isrefreshed, using the method illustrated in FIG. 3 or another suitablemethod. In a next step 506, the value of the counter is incremented.

In a step 508, the processor 270 determines whether the memory refreshoperation is complete. A number of conditions may indicate that thememory refresh operation is complete. For example, the memory refreshoperation may be considered complete when each memory cell 320 in amemory array 310 has been refreshed, or when each memory cell 320 in aparticular block of memory has been refreshed.

If the memory refresh operation is not yet complete, then, in a step510, the processor 270 determines whether system resources are availableto refresh the memory cell 320 at the address corresponding to thecurrent value of the counter. If system resources are available, thenthe process returns to step 504, where the memory cell 320 at thecurrent address is refreshed, and the process continues, as describedabove.

On the other hand, if system resources are not available to refresh thememory cell 320, then, in a step 512, the processor 270 determineswhether the timer has exceeded a predetermined maximum wait time. Asdiscussed above in connection with FIG. 4, the appropriate value for themaximum wait time can be determined by considering a number of factors,such as, for example, the maximum time that a memory cell 320 can retainits assigned state, the time required to refresh a memory cell 320, thenumber of memory cells 320 to be refreshed, and the like. Moreover, asdiscussed above, this predetermined maximum wait time can advantageouslybe a relatively long period of time, such as, for example, a period ofseconds, minutes, hours, days, weeks, or longer.

In one embodiment, the maximum wait time discussed above in connectionwith FIG. 4 corresponds to the maximum time that the processor 270 canwait before it forces system resources to become available to refresh anindividual memory cell 320. In the process illustrated in FIG. 5, on theother hand, the maximum wait time corresponds to the maximum time thatthe processor 270 can wait before it forces system resources to becomeavailable to refresh all of the remaining memory cells 320 in the memoryarray 310 or in the block of memory being refreshed. Thus, in oneembodiment, the maximum wait time is a period of about one day. Inanother embodiment, the maximum wait time is a period of about one week.In yet another embodiment, the maximum wait time is a period of aboutone month to two months.

If the maximum wait time has not yet been reached, then process returnsto step 510, where, as described above, the processor 270 continues tomonitor whether system resources have become available to refresh thememory cell 320. Once the timer reaches the predetermined maximum waittime, if system resources have not become available, then the processproceeds to a step 514, where the processor 270 forces certain systemresources to be relinquished by other processes, such that the necessaryresources become available to refresh the memory cell 320. The processthen returns to step 504, where the memory cell 320 at the currentaddress is refreshed, and the process continues, as described above.

This process repeats until, during step 508, the processor 270determines that the memory refresh operation is complete. Once thisdetermination is made, the process then proceeds to a step 516, wherethe memory address counter is reset. In a final step 518, the processends.

In one embodiment, the process illustrated in FIG. 5 is repeated by theprocessor 270 periodically. In this embodiment, the memory refreshoperation is performed periodically, rather than being a constantlyongoing process, as described above in connection with FIG. 4. Theprocessor 270 can advantageously call and perform this process atregular time intervals or at irregular time intervals, depending uponthe requirements of the computer system 250 and upon the longevity ofthe data stored in the memory cells 320.

The processes illustrated in FIGS. 3, 4 and 5 are merely examples ofalgorithms that can be implemented in the memory refresh instructions290. Those of ordinary skill in the art will understand that theseexemplary algorithms can be easily modified by adding, removing, orvarying certain steps. Moreover, in light of the present disclosure,those of ordinary skill in the art will understand how to develop a widevariety of alternative algorithms.

1. A method of preserving data stored in a memory element having aplurality of variable resistance memory cells, said method comprising:addressing a first individual memory cell of said plurality of variableresistance memory cells; waiting for a first period of time since saidfirst individual memory cell was addressed; determining whether saidfirst individual memory cell needs to be refreshed; if said firstindividual memory cell needs to be refreshed, determining whether systemresources are available to refresh said first individual memory cell; ifsaid first individual memory cell needs to be refreshed and if saidsystem resources are not available, monitoring whether said systemresources become available to refresh said first individual memory cellwithin a second period of time since said first individual memory cellwas addressed; if said first individual memory cell needs to berefreshed and if said system resources do not become available withinsaid second period of time, forcing said resources to be relinquished,such that said resources become available to refresh said firstindividual memory cell; and if said first individual memory cell needsto be refreshed, refreshing said first individual memory cell using saidavailable system resources.
 2. The method of claim 1, wherein said firstperiod of time is within the range of approximately one second toapproximately one week.
 3. The method of claim 1, wherein said secondperiod of time is within the range of approximately one week toapproximately two months.
 4. The method of claim 1, wherein saidplurality of variable resistance memory cells comprises a plurality ofprogrammable conductor random access memory cells.
 5. The method ofclaim 4, wherein each of said plurality of programmable conductor randomaccess memory cells comprises: a cell body having a top surface, whereinsaid cell body comprises a chalcogenide-metal ion glass; and twoelectrodes disposed at said top surface, wherein said electrodes arespaced a distance apart from one another.
 6. The method of claim 5,wherein said chalcogenide-metal ion glass comprises germanium selenidehaving a Group IV metal dissolved therein.
 7. The method of claim 6,wherein said chalcogenide-metal ion glass comprises Ag/Ge3Se7.
 8. Amemory system comprising: a processor coupled to a memory device, saidmemory device having a plurality of variable resistance memory cellsthat must be periodically refreshed; wherein said processor isconfigured to wait for a first period of time after a first individualmemory cell is accessed to determine whether said first individualmemory cell needs to be refreshed; and to refresh said first individualmemory cell after said first period of time, if it is determined thatsaid first individual memory cell needs to be refreshed.